The present invention relates to imaging apparatuses and imaging drums for use in an imaging apparatus that forms an image on printing plates or uses a printing plate to apply ink to a receiver media to form an image. In particular, the present invention relates to an imaging apparatus and imaging drum having automatic plate registration detection.
Contact printing remains the most economical method for printing a large number of copies of an image. Contact printing presses utilize printing plates to apply ink to a receiver media to form an image on the media. In this regard, the printing plates have a printing surface with a pattern of markings representing the image to be printed. Prior to printing, the printing plate is fixed to a plate mounting surface. During printing, ink is applied to the printing surface and the printing surface is brought into contact with a receiver media such as paper. An ink pattern is thereby transferred to the receiver media forming an image on the receiver media.
It is essential to ensure that the contact printing plate is properly aligned with the receiver media during printing. To accomplish this, it is necessary to properly align the printing plate on the mounting surface and to properly adjust the position of the printing plate on the plate mounting surface. A printing plate that is properly aligned and positioned is known in the art as being xe2x80x9cin registration.xe2x80x9d
In certain types of printing, more than one printing plate is used to apply ink to form an image on the receiver media. Typically, each printing plate applies a differently colored ink to the receiver media. In this way, the image formed on the receiver media can contain different colors. It will be understood that each printing plate must be in registration when the color image is formed. If the printing plates are not in registration, the image will appear out of focus and the colors in the image will be incorrect.
Before the printing plate is used for printing, an image is formed on the printing plate. A printing plate imaging apparatus is used to form the image on the printing plate. It is necessary to properly register the printing plate during this process. If a printing plate is not in registration when an image is formed on the printing plate, then the printing plate will not generate proper images when it is used for printing.
Thus, there is a need in the art for an imaging apparatus having an image mounting surface adapted to detect registration of a printing plate on an imaging surface.
There have been various attempts to meet this need in the prior art. For example U.S. Pat. No. 5,992,325 shows a method and device for automatically detecting the location of at least one printing plate edge on a plate mounting surface. As shown in this patent, a plate cylinder for a printing press includes a linear array of clamps that hold a trailing or leading edge of the printing plate. Mechanical sensors in the clamps trigger electrical switches. The switches provide an electrical indication of whether the trailing or leading edge of printing plate is positioned within the clamps. When the edge of the plate is detected at each clamp in the array, the plate is considered to be in registration.
U.S. Pat. Nos. 3,595,567 and 4,127,265 show similar structures for determining whether a sheet of receiver media is in registration with a drum. Each of these patents shows two linear arrays of clamps or grippers positioned on opposite sides of the drum. This allows two sheets of material to be held to the drum thus requiring two sets of sensors to detect the alignment of the sheets. U.S. Pat. No. 2,145,520 shows a mechanical structure to accomplish this result.
U.S. Pat. No. 5,479,859 shows a method and apparatus for controlling an automated printing plate change process in a sheet-fed offset printing machine. In this patent, a pair of clamps are used to hold a printing plate in registration. The clamps are arranged in a linear fashion on the drum and have register pins to grip printing plates against a gripping portion. The register pins are electrically insulated from the gripping portion and engage notches in the plate. When the printing plate is in proper registration, bottom edges of the notches seat on and contact the register pins. This provides a conductive electrical path from the register pins to the gripping portion. By sensing whether such a conductive path is formed at each register pin, it becomes possible to determine whether the printing plate is in registration. Thus, in the drum of the ""859 patent, a separate electrical path is defined between each register pin and each gripping portion. This requires an emitter of an electrical signal and a receiver of an electrical signal at each clamp. Further, in the ""859 patent, the alignment is established by the mechanical positioning between the notches on the leading edge of the plate and the register pins.
Thus, the prior art relies upon the detection of the leading or trailing edge of the printing plate and determines whether the printing plate is in registration based upon the signals received from two or more sensors aligned along a single direction that is parallel with either the leading or trailing edge of the printing plate. Such systems can fail to detect lateral mis-position of the printing plate. Further, such systems are not easily adaptable to accommodate different sizes of printing plates. This is because smaller printing plates may not extend across the linear direction of the clamps so that each clamp contacts a portion of the leading or trailing edge of the plate. In such a circumstance, a drum of the prior art will erroneously indicate that a smaller printing plate is not in registration.
Accordingly, what is needed is an imaging apparatus and a mounting surface for use in an imaging apparatus that overcome the limitations of the prior art.
In accordance with the present invention, an imaging apparatus is provided for forming images on an electrically conductive printing plate. The imaging apparatus has a mounting surface having at least three electrically isolated conductors arranged so that when the electrically conductive printing plate is in registration on the mounting surface, the electrically conductive printing plate defines an electrical connection between all of the electrical conductors and, an electrical circuit adapted to sense an electrical connection between all of the conductors and to thereupon generate an output signal.
In accordance with another embodiment of the present invention, a mounting surface for receiving electrically conductive printing plates is provided. The mounting surface has at least three electrically isolated conductors arranged so that when the electrically conductive printing plate is in registration on the mounting surface the electrically conductive printing plate defines an electrical connection between all of the electrical conductors and an electrical circuit adapted to sense an electrical connection between all of the conductors and to thereupon generate an output signal.